Anyone hoping to reverse the loss of oxygen that is creating dead zones throughout the world’s oceans might have to wait a long time for their hopes to materialize.

How long? Like millions of years.

Once oxygen disappears in the seas — a state known as anoxia — changes in the atmosphere will eventually trigger forest fires that will spark a chain of events leading to oxygen returning after millenniums, according a study in the journal Nature Communications authored by researchers at the University of Exeter in Britain.

“It’s going to take a long, long time for the Earth’s system to rebalance,” said study co-author Sarah Baker, a Ph.D candidate in geography at Exeter. “It’s not a flip of the switch thing. It’s not going to take a day a week or a few years.”

Scientists have determined that oceanic oxygen levels are decreasing, killing fish and other life in the Gulf of Mexico, the Baltic Sea, and elsewhere. Agricultural runoff that feeds algae that becomes food for oxygen-sucking bacteria when it decomposes and warmer temperatures that make it harder for water to chemically absorb oxygen from the atmosphere are prime causes of the drop-off.

Making a connection between volcanic eruptions 183 million years ago that emitted massive quantities of carbon into the air — causing deadly drops in oxygen levels in the ocean and massive marine extinctions — Baker and her colleagues studied layers of fossilized charcoal in Wales and Portugal from the same period to see how the land might have reacted.

Their findings depict mechanisms that take place on a geological time scale.

The researchers surmised that carbon spikes in the water after a so-called “anoxic event” like a catastrophic volcanic eruption would cause oxygen levels in the ocean to collapse, spurring the growth of marine plants. More plants would then theoretically produce more oxygen that would eventually make it into the atmosphere.

Confirming their thesis, the fossil record showed an uptick in wildfires around 1 million years after the volcano eruptions. More oxygen in the air correlates to fires, said Baker.

Taking their theorizing a step further, they inferred that more wildfires would have cleared the land of vegetation that helps erode rocks that naturally provide marine nutrients like phosphorus — an ingredient in dish detergents and fertilizers that causes aquatic algae blooms today. The lack of those nutrients would then depress plant populations in the water, robbing fish and other creatures of food. Fewer organisms breathing oxygen would result in oxygen levels rising in the water.

Around 800,000 years later, the heightened wildfire activity ebbed, the researchers found, suggesting the oxygen-carbon balance on the planet had returned to balance.

Baker admitted that the fossil record was just one piece of an enormous puzzle. But the fossils confirmed 14-year-old research that reached similar conclusions using mathematical models, she said.

Pennsylvania State University Geoscientist Katherine Freeman had questions about the findings. It’s not clear what kind of vegetation was burning during the periods that Baker and her fellow researchers had studied. Certain plants could have produced more charcoal during normal wildfire seasons.

But Freeman said the study nonetheless was ammunition for those who argue that humans should emit fewer greenhouse gases into the atmosphere and do more to safeguard the oceans that gave birth to life in the first place.

“It’s a really intriguing suggestion,” said Freeman. “It’s sobering to think of the implication in the modern context.”